1. Field of the Invention
The present invention relates to an electron tube used as a photodetector for quantitatively measuring weak light and particularly having a sensing device such as a semiconductor device for multiplying photoelectrons emitted from a photocathode and outputting the electric signals.
2. Related Background Art
Conventionally, an electron tube which causes an electron lens to accelerate and focus electrons emitted from a photocathode upon incidence of light and makes the photoelectrons incident on, e.g., a semiconductor device to obtain a high gain is known. This electron tube is disclosed in, e.g., U.S. Pat. No. 5,120,949, Japanese Patent Laid-Open No. 6-318447, U.S. Pat. Nos. 5,374,826 or 5,475,227. Particularly, U.S. Pat. No. 5,475,227 discloses a structure for preventing a phenomenon that ions generated from gas molecules adsorbed on the electron incident surface of the semiconductor device due to electrons incident on the semiconductor device are accelerated and fed back to the photocathode to result in a large degradation in photocathode. More specifically, a semicylindrical ion deflecting electrode is arranged immediately before the semiconductor device to bend the orbits of ions generated on the electron incident surface of the semiconductor device, thereby preventing the ions from returning to the photocathode.